The present invention generally relates to optical fibers and more particularly, to an optical connector for single-mode optical fibers required to be coupled with each other with high accuracy and a method of manufacturing a pair of ferrules for the optical connector.
Conventionally, in a single-fiber connector for a single-core coated fiber 1, it has been so arranged as shown in FIGS. 1a and 1b that a cylindrical ferrule 3 made of metal has a flange 6 and is formed, at its axis, with a stepped through-hole, i.e., a hole 5' and an aperture 5 having a diameter smaller than that of the hole 5' and extending in alignment with the hole 5'. The single-core coated fiber 1 is constituted by one single-mode optical fiber having a coating therearound, while an exposed fiber (glass portion) 2 is obtained by removing the coating from an end portion of the coated fiber 1. The coated fiber 1 and the exposed fiber 2 are, respectively, fitted into the hole 5' and the aperture 5 until the coated fiber 1 is blocked by a bottom face of the hole 5'. Subsequently, adhesive 4 is filled into a gap between the coated fiber 1 and the hole 5' and thus, the coated fiber 1 is secured to the ferrule 3. Thereafter, a portion of the exposed fiber 2, which projects out of a corresponding end face of the ferrule 3, is cut off from the exposed fiber 2 and then, the cut edge of the exposed fiber 2 received by the aperture 5 is finished by grinding so as to be flush with the end face of the ferrule 3 such that the end face of the ferrule 3 acts as a coupling face of the ferrule 3. Then, a sleeve having an inside diameter identical with an outside diameter of the ferrule 3 is prepared. Thus, a pair of the ferrules 3 each having the coated fiber 1 secured thereto are inserted into opposite ends of the sleeve, respectively until the respective coupling faces of the ferrules 3 are brought into contact with each other, so that the optical connector is obtained.
Meanwhile, for example, a single-mode optical fiber has an outside diameter of 125 .mu.m but its core diameter is merely 10 .mu.m, far smaller than the outside diameter of the optical fiber. Accordingly, the optical fiber is required to be positioned with an accuracy of not more than 1 .mu.m. For example, in order to obtain a transmission loss of not more than 1 dB at the optical connector, an eccentricity of the aperture 5 relative to an outside diameter of, e.g., 2.5 mm of the ferrule 3 is required to be restricted to 1 .mu.m at its maximum, thereby resulting in an extreme rise in production cost of the prior art optical connector.
Furthermore, even if the eccentricity of the aperture 5 relative to the outside diameter of the ferrule 3 is restricted to not more than 1 .mu.m, the transmission loss at the optical connector exceeds 1 dB when an eccentricity of a core of the optical fiber relative to a cladding of the optical fiber is more than 1 .mu.m. Therefore, in order to prevent such a phenomenon, the outside diameter of the ferrule 3 is required to be subjected to grinding coaxially with the core of the exposed fiber 2, but such grinding leads to low productivity rate and high production cost of the known optical connector.
Meanwhile, as shown in FIGS. 2a and 2b, in a prior art multi-fiber connector for a multicore coated fiber 1 having multiple, e.g., five optical fibers, a rectangular ferrule 7 is formed with a pair of guide holes 8, each for receiving a coupling pin. In the same manner as the known ferrule 3 of FIG. 1, the multiple optical fibers are, respectively, fitted into stepped through-holes arranged linearly so as to be disposed between the guide holes 8 and then, the coated fiber 1 is secured to the ferrule 7. Thereafter, the end face of the ferrule 7, out of which the exposed fibers 2 project, are ground into a coupling face of the ferrule 7. After the respective coupling faces of a pair of the ferrules 7 have been brought into contact with each other, a pair of the coupling pins are inserted through one pair of the confronting guide holes 8 of the confronting ferrules 7 and the other pair of the confronting guide holes 8 of the confronting ferrules 7, respectively so as to position the confronting ferrules 7. Subsequently, the confronting ferrules 7 positioned by a pair of the coupling pins are clamped by a clamp housing and thus, the optical connector is obtained. However, this known optical connector has such a drawback that the apertures 5 for receiving the exposed fibers 2, respectively are required to be machined with high positional accuracy relative to the guide holes 8, thereby resulting in rise of its production cost.
Moreover, the known ferrules 3 and 7 require areas of about 5 mm.sup.2 and about 10 mm.sup.2 per optical fiber, respectively, which are approximately ten times a cross-sectional area of about 0.6 mm.sup.2 of the coated fiber 1, thus preventing the known optical connectors of FIGS. 1 and 2 from being made compact in size.
It should be noted here that the exposed fiber 2 obtained by removing the coating from the coated fiber 1 means generally a "bare optical fiber" and removal of the coating means "removal of only a coating for making positioning of the optical fiber unstable". Accordingly, for example, a thin film of 1 to 10 .mu.m in thickness coated on a surface of the bare optical fiber is not required to be removed. Furthermore, even if a coating layer has a thickness of as large as 200 .mu.m, it is needless to say that the coating layer is not required to be removed if the coating layer is sufficiently brought into close contact with the bare optical fiber without adversely effecting positioning of the optical fiber at all.